The productivity of oil and gas wells can be improved by increasing the area of communication within a selected producing zone. The drainage area can be increased by hydraulic fracturing of the producing zone to provide fractures and channels emanating from the well base area into the contiguous subterranean formations.
The hydraulic fracturing process is accomplished by rapid pumping of an aqueous fluid medium down a well which penetrates the subterranean formation where fracturing is desired. The rapid pumping of the aqueous fluid creates a hydrostatic pressure which energizes splitting forces in the confined zone. Pressures as high as 10,000 psi are employed to effect formation fracturing.
As cracks and channels are formed, a propping agent which is suspended in the high viscosity hydraulic fluid penetrates the newly created fissures and becomes lodged therein. The function of the propping agent is to support the fractures in an open position as a conduit for the flow of fluids such as oil, gas or water through the fractured zone. Various noncompressible materials are employed as proppants. These include sand, rounded walnut shells, glass beads, aluminum spheres, and the like. U.S. Pat. No. 3,708,560 proposes polyester resin prills as a proppant in hydraulic fracturing fluid systems.
After a fracturing operation has been completed and the propping agent has been deposited, the hydrostatic pressure is released, the flow of fluid is reversed, and the hydraulic fracturing fluid is withdrawn.
Hence, the hydraulic fracturing fluid composition functions to force fracturing under hydrostatic pressure, and it serves to transport the suspension of propping agent into the porous subterranean formations. The hydraulic well-treating fluid medium must exhibit advantageous viscosity and particulate solids transport properties.
Well-treating hydraulic fluids commonly employ a viscosity increasing agent such as a water-soluble polymer to improve the flow characteristics and the suspending ability of the fluids. Carboxymethylcellulose, polyacrylamide, polyvinylpyrrolidone, guar, tragacanth, Irish moss, modified starch, and the like, have been employed as water-soluble or water-dispersible additives to provide pumpable gels having solids suspending properties. U.S. Pat. Nos. 3,483,121; 3,757,864; 3,779,914; 3,818,998; 3,900,069; 3,971,440; and references cited therein, describe well-treating hydraulic fluid compositions which contain novel polymeric friction-reducing gelling components.
U.S. Pat. No. 3,516,496 describes an improved well-completion and workover hydraulic fluid composition comprising an aqueous solution containing a synergistic additive mixture of a polyoxyethylene polymer and powdered marble. The composition is injected into permeable subterranean formations to plug portions thereof temporarily by means of an impermeable filter cake. The filter cake is subsequently removed by backflowing and/or dissolving the solid particles with a pH-adjusting fluid such as hydrochloric acid.
U.S. Pat. No. 3,836,465 describes a viscoelastic hydraulic fluid composition comprising an aqueous medium containing a mixture of polyalkylene oxide and lignosulfonate salt. The composition is recommended as a fluid loss control agent in drilling fluids and as a diverting agent in subterranean strata.
Other well-treating hydraulic fluid compositions are described in U.S. Pat. Nos. 3,765,918; 3,882,029; 3,953,336; 3,988,246; 4,048,079; and 4,038,206. These particular prior art hydraulic fluids have in common the use of a xanthomonas gum as a gelling agent, particularly in combination with another gelling agent. When xanthomonas gum is blended with another hydrocolloid in an aqueous medium, the resultant solution normally exhibits a viscosity which is consistent with the additive viscosities of the two components.
Hydraulic fracturing fluids which contain a natural resin, such as xanthomonas gum or hydroxyethylcellulose, commonly include a breaker additive to effect a delayed action thinning out of the fluid medium. This facilitates the subsequent removal of the hydraulic fracturing fluid composition from the fractured formation.
Breaker additives include enzymes which under the well-fracturing conditions in a delayed manner autonomously reduce the hydraulic fluid viscosity by degrading the natural gum incorporated as a gelling agent. U.S. Pat. Nos. 2,681,704; 2,801,218; 3,615,794; 3,684,710; and references cited therein, describe new enzyme compositions, such as mannan depolymerase which is derived from a microbiologic source.
The disadvantage of employing natural gum as a gelling agent in hydraulic fracturing fluids is the lessening of fluid loss control caused by plugging of apertures which reduces the permeability of a fractured formation. In the case where a natural resin is employed as a gelling agent in combination with an enzyme breaker additive in a hydraulic fracturing fluid, the hydraulic gel system is often unsatisfactory because the hydrolysis residue of the natural gum is present in sufficient quantity to create a plugging effect and reduces the permeability of a fractured formation.
There remains a need for an efficient and economic hydraulic fluid medium which exhibits improved properties in well-treating applications.
The use of hydrocolloids and mixtures of hydrocolloids is also common in various other formulations developed for specific applications in diverse industries.
U.S. Pat. No. 2,868,741 describes a water base stencil duplicating ink composition which includes a mixture of hydrocolloids such as polyacrylic acid and hydroxyethylcellulose.
U.S. Pat. No. 3,325,425 describes a carboxy-containing acrylic interpolymer latex paint formation which includes a hydrocolloid thickener mixture such as methylcellulose and sodium salt of polycarboxylated condensed naphthalene.
U.S. Pat. No. 3,793,272 describes a synthetic resin composition which consists of an emulsion or latex of a vinyl or acrylic resin such as polyacrylic acid in admixture with a hydroxybutylhydroxyethylcellulose derivative.
As is evident from the prior art exemplified hereinabove, hydrocolloid thickened fluid media find important technical application in a broad variety of manufacturing industries. There is continuing research effort to develop novel high viscosity aqueous systems which exhibit improved properties in diverse applications.
Accordingly, it is a main object of this invention to provide a thixotropic fluid composition comprising an aqueous medium and a synergistic mixture of hydrocolloid components, which aqueous composition exhibits a greater viscosity than the sum of the calculated additive viscosity values of the said hydrocolloid components.
It is another object of this invention to provide a partially-synthetic hydrocolloid thickened aqueous medium having improved viscosity and suspending properties for application as a well-treating hydraulic fluid composition.
It is a further object of this invention to provide a well-fracturing hydraulic fluid composition containing a partially synthetic hydrocolloid gelling agent in combination with a breaker additive, which is characterized by a low yield of hydrolysis residue.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.